Engineered lignocellulosic-based panels, such as oriented strandboard, high-density fiberboard, medium density fiberboard, chipboard, particleboard, hardboard, laminated veneer lumber and plywood, are commonly used as roof, wall and floor sheathing in the construction of buildings and residential homes. A significant portion of this construction occurs outdoors at the building site. Thus, the engineered lignocellulosic-based panels are vulnerable for a period of time to rain or snow. It is well known that exposure to water can cause engineered lignocellulosic-based panels to undergo dimensional expansion. For instance, many engineered lignocellulosic-based panels will swell in thickness by a factor that is substantially greater than that experienced in the width and length dimensions and that swell is often inelastic in response to a wet/redry cycle. Thus, engineered lignocellulosic-based panels have a tendency to expand in thickness during their first exposure to water, and if the panel is later dried, the thickness dimension might decrease to some extent, but it does not return to its original value. Thus, the builder is faced with the dilemma of coping with roof, wall and floor surfaces that are geometrically irregular.
A second problem that often occurs when engineered lignocellulosic-based panels are exposed to water is a reduction in strength or structural load-carrying capacity. In addition to exposure to water during construction, exposure to water can also occur during occupancy of the structure. For example water can be introduced into the structure by wind-driven rain, which can be forced through leaks around various structure elements, such as doors, windows and roofs. Inadequate seals in water pipes can also cause engineered lignocellulosic-based panels to be exposed to water. Additionally, recent construction practices tend to result in buildings with reduced levels of ventilation. This condition can cause the accumulation of moisture inside of buildings, especially in wall cavities, crawl spaces and attics. The ability of the engineered lignocellulosic-based panels to withstand these insults for some extended period of time without significant loss of structural properties or the development of mold or incipient decay is an important quality.
Companies that manufacture engineered lignocellulosic-based panels have recognized the problems associated with exposure to water for many years. In an effort to improve the properties of engineered lignocellulosic-based panels in a wet environment a number of technologies have been developed and implemented. For instance, wax is typically incorporated into engineered lignocellulosic-based panels in order to retard the penetration of water. Also, most engineered lignocellulosic-based panels are treated on the edges with a sealant, which helps the panel to resist the absorption of water at the edges where thickness swell is most prominent and problematic.
It is generally believed that many of the properties associated with engineered lignocellulosic-based panels could be improved if higher binder levels were used. Unfortunately, a variety of constraints make it difficult for engineered lignocellulosic-based panels manufacturers to utilize higher binder levels.
To overcome these problems U.S. Pat. No. 3,632,734 described a non-conventional method for manufacturing engineered lignocellulosic-based panels. This patent describes a method for reducing swelling in engineered lignocellulosic-based panels that is based on the following key steps: a phenol-formaldehyde impregnating resin is applied to green wood particles at a level of about 4–8%; the treated green wood particles are dried under temperature conditions that avoided pre-cure of the impregnating resin; a phenol-formaldehyde resin binder is then applied to the dried wood particles at a level of about 4–8%; and the treated particles are formed into a mat and subjected to heat and pressure to form a panel and cure the resins.
It should be noted that urea is typically added to phenol-formaldehyde resin binders in an attempt to limit emissions of formaldehyde. When urea is heated as the green strands are subsequently dried at elevated temperatures, the urea produces an ammonia emission. The ammonia emission can result in a NOx emission if the ammonia is processed through a pollution control device known as a Regenerative Thermal Oxidizer (RTO). There are regulatory limitations associated with such NOx emissions. If the plant does not have an RTO, or some other heat system that puts resin emissions through a burner, there will be no NOx formed, although in that case ammonia would still be emitted to the atmosphere.
More recently, U.S. Pat. No. 6,572,804 discloses the application of a phenol-formaldehyde resin to green strands and subsequent drying of the strands in the presence of methyol urea. The dry treated strands are optionally blended with more binder and are eventually consolidated under heat and pressure to yield a building panel. The patent discloses a new phenol-formaldehyde resin binder that is produced by adding urea to a liquid phenol-formaldehyde resin and subsequently adding formaldehyde to the same resin in order to convert the free urea into methyol urea. The patent claims that the new phenol-formaldehyde resin binder is less likely to emit ammonia than a conventional phenol-formaldehyde resin binder that was made with only a post addition of urea. Unfortunately, the methyol urea adduct has the potential to emit significant levels of both ammonia and formaldehyde when it is heated.
Thus, there continues to be a need for engineered lignocellulosic-based panels with improved performance in the presence of water. It is recognized that such a panel could be made by use of “green-strand-blending”. However, in order to satisfy emission requirements, the resin used in the green-strand-blending process must not emit significant levels of ammonia or volatile organic compounds, including formaldehyde, phenol and methanol.